In order to achieve a very high output electrical power of the fuel cell, which is tantamount to generating an electric current of large current strength, a high cathode pressure is required. In devices of the prior art, one works with an elevated mass flow of the air compressor; a throttle valve in the outlet line or a regulated bypass valve spanning the fuel cell regulates the cathode pressure of the fuel cell to the desired value.
A relatively high cathode pressure is also required for working at lesser output power in order to satisfy the stoichiometry in the individual cells of the fuel cell. When a bypass valve spanning the fuel cell is used, the maximum cathode pressure can be achieved by closing the bypass valve, for then the total air mass is taken solely to the fuel cell, and/or by increasing the rotational speed of the compressor. However, the value then obtained for the cathode pressure is limited by the design of the technical features of the fuel cell which bring about the pressure drop. The efficiency of the fuel cell becomes less when it is supplied by the compressor with a relatively high air flow.
In JP 2005310429 A a regulator 54 is provided for the generated power, which apparently constitutes an AC/DC converter. This is connected to a battery. The drawing shows an electrical connection from this battery to the compressor/air supply unit. The battery is energized only via the generator. Its power cannot suffice to meet the high power demand of the compressor (if the latter is electrically operated). This architecture is only technically meaningful if the battery is furthermore recharged by a high-voltage circuit.
In JP 2006286559 A a similar architecture is proposed, showing basically two electrical architecture variants. In variant 1, the electrical compressor unit and the exhaust gas turbine are connected via the controller 4 to the high-voltage circuit. In variant 2, the compressor unit and the exhaust gas turbine are connected via a controller 8 to a common energy storage 7 (battery, capacitor bank, etc.). Here as well, this architecture is only technically meaningful if the energy storage 7 is recharged by the high-voltage circuit.
If one wishes to optimize the efficiency of a fuel cell, especially a hydrogen fuel cell, much attention needs to be paid to the electrical energy put into the system, which must be as low as possible. According to the present prior art, a turbocharger with electrical drive and a radial compressor with impeller are used at the input side for the air supply. At the output side, a radial turbine is used, which is driven by the outflowing air. This is mechanically connected to rotate with the shaft of the impeller. The drawback of this arrangement is the lower degree of freedom for the turbine to work in its optimal range regardless of the range of the compressor. One consequence of this is that the measure of recovered kinetic energy is relatively low in the event of slight power demand of the fuel cell from the driving motor of the motor vehicle and relatively low air flow. Furthermore, it is not possible to employ a control of the backpressure of the air other than by using an additional component, such as a control valve at the air outlet of the fuel cell or a system bypass or variable turbine geometry, as indicated in several applications of the prior art. Each of these solutions has a limited degree of free opportunities for controlling the cathode pressure or achieving a higher efficiency.